Aging is universal and the most potent risk factor for cardiovascular disease (CVD). Aortic stiffness increases with age in both sexes and predicts CVD risk. Stiffening of the aorta raises central pulse pressure to contribute to hypertension, heart attack and stroke and promotes small vessel remodeling that damages end organs inducing heart failure, renal dysfunction, and cognitive decline. Thus, understanding aging vascular stiffness mechanisms is a pressing unmet medical need. We recently discovered that inhibition of the aldosterone- binding mineralocorticoid receptor (MR) or smooth muscle cell (SMC)-specific deletion of MR (SMC-MR-KO) protects male mice from vascular stiffness with aging and now propose to explore mechanism(s). Women develop vascular stiffness later in life, faster, and with significantly worse CVD outcomes than men. New data reveals a similar pattern of later onset vascular stiffness in aging female mice that correlates with the timing of increased aortic SMC-MR expression. SMC-MR deletion protects from vascular stiffening in both sexes by sexually dimorphic mechanisms; SMC-MR contributes to vessel fibrosis in aging males and to intrinsic SMC stiffness (by atomic force microscopy (AFM)) in both sexes. We show for the first time that MR protein increases with age in primary human aortic SMCs (HASMC) from males and females which may be driven by induction of the HIF1a transcription factor. Additional data from male mouse aortas reveals: (1) Decreased expression with age of fibrosis genes and integrin receptors specifically in SMC-MR-KO mice; (2) Decreased histone H3K27 methylation (H3K27me) with aging that is attenuated in SMC-MR-KO mice; (3) Decreased expression of the H3K27 methyltransferase EZH2 with age with enrichment of H3K27ac, a transcriptional activation mark, at promoters of fibrosis genes only in MR-intact mice. Analogous to the mice, aged male HASMC had decreased EZH2 and H3K27me and increased stiffness genes, and these changes are reversed by MR antagonism. Based on these data, we propose to test the hypothesis that: rising MR in aging SMC promotes stiffness by suppression of EZH2 leading to decreased H3K27me and recruitment of MR and it's co- activator CBP to increase H3K27ac and transcription of stiffness genes. SA1 determines mechanisms for increased MR expression in aging SMCs. SA2 investigates MR as an epigenetic regulator of stiffness genes in HASMCs in vitro. SA3 explores MR as an epigenetic regulator of vascular stiffness in aging mice. All studies directly compare male and female HASMCs or mice with gain- and loss-of-function genetic and pharmacologic approaches to target MR, EZH2, and CBP. The proposal uses innovative animal models and state-of-the-art approaches to explore a novel epigenetic mechanism driving aging vascular stiffness and tests sex-specific mechanisms by which SMC-MR contributes. The long-term goal is to determine the therapeutic potential for antagonism of MR or its downstream mechanisms as sex-specific precision medicine strategies to mitigate aging-induced vascular stiffness and the myriad of adverse consequences in our rapidly aging population.